Single-chip electronic noses, enabled by full on-chip integration of gas chemical microsensors with signal-conditioning electronics have tremendous medical, environmental and safety applications. Gravimetric detection is an important sensing modality for these microsystems.
Commercially available mass-sensitive devices for volatile organic compound detection use piezoelectric quartz substrates. Thickness shear mode resonators (TSMR), also known as quartz micro-balances (QMB) (Patel, R., Zhou, R. Zinszer, K., and F. Josse, “Real-time detection of organic compounds in liquid environments using polymer-coated thickness shear mode quartz resonators”, Analytical Chemistry, vol. 72, no. 20, p. 4888-4898, 2000) (Schierbaum, K. D., Gerlach, A., Haug, M., and W. Gopel, “Selective detection of organic molecules with polymers and supramolecular compounds: application of capacitance, quartz microbalance, and calorimetric transducers”, Sensors and Actuators A, 31, p. 130-137, 1992), and Rayleigh surface acoustic wave (SAW) devices (Ricco, A. J., Kepley, L. J., Thomas, R. C., Sun, L., and R. M. Crooks, “Self-assembling monolayers on SAW devices for selective chemical detection”, IEEE Solid-State Sensor & Actuator Workshop, Hilton Head, S.C. June 22-25, p. 114-117, 1992) are examples of such devices. However, these piezoelectric devices have not been fully integrated with on chip electronics. In contrast, resonant cantilever chemical microsensors integrated with CMOS have been demonstrated (A. Hierlemann and H. Baltes, “CMOS-based chemical microsensors”, Analyst, 128, p. 15-28, 2003). Prior work on cantilever mass sensors includes detection of humidity, mercury vapor, and volatile organic compounds (Lange, D., Hagleitner, C., Hierlemann, A., Brand, O., and H. Baltes, “Complementary Metal Oxide Semiconductor Cantilever Arrays on a Single Chip: Mass-Sensitive Detection of Volatile Organic Compounds”, Analytical Chemistry, vol. 74., no. 13, p. 3084-3095, 2002) as well as biomolecular recognition in a liquid media (Fritz, J., Bailer, M. K., Lang, H. P., Rothuizen, H., Vettiger, P., Meyer, E., Guntherodt, H. J., Gerber, Ch., and J. K. Gimzewski, “Translating biomolecular recognition into nanomechanics”, Science, 288, p. 316-318, 2000.). Post-CMOS micromachining has been used to make fully integrated mass sensitive oscillators with pico-gram resolution (H. Baltes, D. Lange, A. Koll, “The electronic nose in Lilliput,” IEEE Spectrum, 9, 35, (1998)). These devices were formed through deposition of precise amounts of a chemically sensitive layer onto relatively wide cantilevers.
Another example of a CMOS-MEMS resonant gas sensor used electrostatic actuation and detection to form a free-running oscillator (S. S. Bedair and G. K. Fedder, “CMOS MEMS Oscillator for Gas Chemical Detection,” Proceedings of IEEE Sensors, Vienna, Austria, Oct. 24-27, 2004). A cantilever beam suspended a plate made large enough to accommodate drops of chemically sensitive polymer placed directly onto the plate using drop-on-demand ink jet deposition. Ink jet deposition can functionalize each cantilever in an arrayed structure with a separate polymer. This non-contact technology is scalable for large arrays, easy to use, versatile, and faster than other means of coating such as from micro-capillaries and drop casting from pipettes (A. Bietsch, J. Zhang, M. Hegner, H. P. Lang, and C. Gerber, “Rapid functionalization of cantilever array sensors by inkjet printing”, 2004 Nanotechnology 15 873-880). Other thin film application methods include dip pen and shadow mask processing which are both time consuming processes.
Another prior microfluidic system is described in U.S. patent application, 20050064581 and in a corresponding paper (T. P. Burg, A. R. Mirza, N. Milovic, C. H. Tsau, G. A. Popescu, J. S. Foster and S. R. Manalis, “Vacuum-packaged suspended microchannel resonant maass sensor for biomolecular detection,” J. Microelectromechanical Systems, December 2006). These prior art documents describe an enclosed microchannel. Material is flowed into the channel to functionalize sidewalls of the channel to capture biomolecules on the sidewalls. However, the channel in these works is not used or taught as a wicking structure for deposition of a non-liquid material, such as polymers, that fills or partly fills the channel. Specifically, the patent application describes a microfluidic channel to detect analyte that may have a liquid or gel in the channel. The analyte is flowed into the microchannel. The gel may be delivered by pressure flow or electrophoresis, but no description or teaching of gel deposition through wicking is provided. The invention requires an enclosed microchannel for analyte delivery through flow, and in order to package in vacuum.
It is beneficial to further scale down the size of the resonant microstructure to achieve an increased mass sensitivity and reduced cost. Scaling cantilevers down to micro- and nano-scale dimensions is achievable with optical or piezoresistive resonant detection. However, microstructures with low-noise electrostatic actuation and detection require narrow air gaps that are generally incompatible with existing polymer deposition techniques.
Accordingly, there is a need for improved apparatuses and methods to control polymer addition to micro-cantilevers and nano-cantilevers for biological and chemical sensing. Those and other advantages of the present invention will be described in more detail hereinbelow.